Nb3 ga base superconducting materials

ABSTRACT

A superconducting material having a critical temperature higher by 3* to 5*K than that of and a critical current density several times higher than that of Nb3Ga provided by replacing a part of atoms in the Ga sites of Nb3Ga with atoms of one of Al, Mn, Sb, and Y and, when required, replacing a part of atoms in the Nb sites with Ta atoms and by subjecting this intermetallic compound to an aging process at 800*C for 300 hours. This material has a Beta -W type crystal structure similarly to Nb3Ga.

United States Patent 1191 Kawabe et al.

[ Apr. 2, 1974 [54] "NECIEKSEFWNEFCTINF MATERIALS [75] Inventors: Ushio Kawabe; Mitsuhiro Kudo,

both of Tokyo; Shigeo Fukase, Hachioji, all of Japan [73] Assignee: Hitachi, Ltd., Tokyo, Japan [22] Filed: May 19, 1972 [21] Appl. No.: 254,901

[30] Foreign Application Priority Data May 20, 1971 Japan 46-34596 52 US. Cl 75/174, 335/216, 14s/32;133 15 1111. c1 c22 27/00, HOlf 1/04, HOlf 1/14 [58] Fieldot'Search ..75/174;l48/32,133;

OTHER PUBLICATIONS Zeitschrift fur Physik 215, 1968, pages 323-334.

keferenc es'ifitd J 1x 5511211"'rilyaeg'varzog mp5; April 1968, pgs. 2000-2002.

Metallurgy of Advanced Electronic Materials, Brock, Metallurgical Society Conferences, Vol. 19, 1966, pgs. 71 & 8l83.

Primary ExaminerCharles N. Lovell Attorney, Agent, or Firm-Craig and Antonelli [57] ABSTRACT 11 Claims, 8 Drawing Figures AS CAST k: 23

A 20- X a.

w n: E E m o. I 111 F A d 2 m 0 CONCENTRATION OF A9. ATOMS y "MENTEDAPR .21974 SHEEI 1 [1F 8 FIG.

UNIT CELL OF B-W' TYPE CRYSTAL STRUCTURE PMENIEUA-PR 21914 31301.31 3

sum 2 nr 8 FIG. 2

25 I Q I CRITICAL TEMPERATURE (k) CONCENTRATION OF M ATOMS y sum 3 ur 8 FIG. ,3

"- AS CAST k:2

O 0.5 IO

CONCENTRATION OF A9. ATOMS y 'AIENTEDM'R 2m: 3801313 sum 14 or 8 CRITICAL TEMPERATURE k) CONCENTRATION OFM ATOMS 25x) (AT.% M)

Pmrgmimrn 219m 3801.313

sum s I]? a FIG. 5

30 l I I oss 0.05 3 I-y y -0 AS AGED AS CAST CRITICAL TEMPERATURE (I CONCENTRATION OF AlATONS y AIENTEHAPR 2 I974 SHEET 8 0F 8 OOON E301 OOmV PATENTEDAPR 2 m4 SHEEI 8 BF 8 FIG.

o 2 O l A o o u G s b N H l C J m K k 20 4 8 A m m w AGING TEMPERATURE (C) (FOR IHOUR) 1 Non. BASE SUPERCONDUCTING MATERIALS BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION The present invention relates to a superconducting material of an Nb Ga base intermetallic compound having a B-W type crystal structure, and'more particularly to a superconducting material having a high critical temperature and a high critical current density.

2. DESCRIPTION OF THE PRIOR ART Superconducting materials have such interesting properties as perfect conductivity, diamagnetism, transition phenomenon, etc.

Various appliances utilizing these properties, such as superconducting magnets, magnetic shields, cryotrons, and the like have hitherto been developed.

Then, in the B C S theory the value of m is represented as an approximation to a value which is proportional to the order of the Debye temperature of the superconducting material, the value of V is constant, and [N is represented as an approximation to a value which is proportional to the electronic specific heat coefficient (y; of the superconducting material.

Therefore, in order to obtain a superconducting material having a high critical temperature, it is necessary t0 find a material with a high Debye temperature and a high electronic specific heat coefficient. Table 1 shows the critical temperature and crystal structure of some well known superconducting materials which have a considerably high critical temperature.

TABLE 1 BINARY COM POSITION Generally, in order to reveal the superconductivity,

it is necessary to cool the superconducting materials at temperatures lower than their critical temperatures.

The critical temperature (Tc) of Nb sn, which has been considered to be the superconducting material having the highest critical temperature, is 18K.

Consequently, a cryogenic technique which utilizes expensive liquid helium as a coolant is necessary in order to cool these superconducting materials below their critical temperatures.

Under these circumstances the discovery of a superconducting material having even a little higher critical temperature than that of the known superconducting material has been greatly desired.

According to the B C S theory [.I. Bardeen, et al., Phys. Rev., 108 (1957) 1175] relating to superconductivity, the critical temperature Tc of a superconducting ,0

material is represented by the following formula:

Tc w,, exp (l/g) where w is the average angular frequency of the phonons that scatter electrons at the Fermi surface and g is the product of the net attractive interaction between the electrons (V) and the density of states of delectrons [N at the Fermi surface.

Some of the superconducting materials having a relatively high critical temperature are found in the group of intcrmetallic compounds, which have the B-W type crystal structure, the relatively high electronic specific heat coefficient (y), and the high Debye temperature (0 i.e., Nb Ga, Nb Al, Nb Al ,,Ge,,, and the like.

However, such superconducting materials having the BW type crystal structure as mentioned above cannot be obtained until they are subjected to difficult aging such as 700C X 1,000 hours. Thus, the manufacture of these materials is not easy.

On the basis of the McMillans theory presented recently in Phys. Rev., 167 ('68) 331, by W. L. McMillan, the critical temperature of a superconducting material is given by the following formula;

0.62 )t)], where w,, is the average angular frequency of phonons, and p.* and A are the electron-electron and electron-phonon coupling constants, respectively. By Mc- Millans theory, it is concluded that the maximum value of critical temperature of superconducting materials is within a range of 25K to 40K. However, no superconducting matrial having such a high critical temperature as the upper limit as predicted by McMillan of the critical temperature has yet been produced.

The critical temperatures of the intermetallic compounds as aged, represented by a general formula Nb Al, Ga which are provided by replacing a part of the Al atoms in Nb Al with Ga atoms are shown in an article by G. Otto in Zeitschrift ft'ir Physik, 215 (I968) 328. It is reported in this article that a superconducting material having a critical temperature of l9.lK was provided by an aging process-at 800C for 10 hours when the atomic concentration x of Ga was 0.33. However, according to the article by Otto, the peak value of the critical temperature is present on the Nb Al side only, and no peak is present on the Nb Ga side, that is, on the Nb Ga side the critical temperature monotonically increases with the amount of Al.

The ternary compound on the Nb Al side at which the peak of the critical temperature is present is mechanically very hard and brittle, and the material as cast has many cracks and hence is practically unusable. For example, the Knoop hardness of Nb Al Ga as cast is 895Kp/mm SUMMARY OF THE INVENTION An object of the present invention is to provide a superconducting material having a novel composition which can be made into a stable B-W type crystal structure and having a high critical temperature even by a relatively short period of treatment in contrast to the previously known superconducting materials as having relatively high critical temperatures which requires a considerable period of aging.

Another object of the present invention is to provide a superconducting material having a high critical temperature and a relatively high critical current density (10).

Another object of the present invention is to provide a superconducting material to be used under simpler cryogenic conditions than previously.

The superconducting material according to the present invention is provided by improving Nb Ga which is known as one of the superconducting materials having high critical temperatures among the group of the superconducting binary intermetallic compounds having the B-W type crystal structure.

The superconducting materials of the present invention are characterized in that a suitable amount of Ga of the Nb Ga is substituted by a third element M which has a larger atomic radius of a coordination number 12 than that of Ga, and which simultaneously has a higher Debye temperature than that of Ga. The third element is an element selected from the group consisting of aluminum (Al), antimony (Sb), manganese (Mn), and yttrium (Y).

Occasionally, according to the present invention a suitable amount of Nb of the material Nb Ga is further substituted by tantalum (Ta).

Therefore, the chemical composition of the superconducting material of the present invention is represented by the following general formula;

1.r .1-)k -1t/ w where the values of x, y, and k are selected to be within such ranges that it is easy to crystallize the composition into a desired B-W type crystal structure and hence heat treatment is also easy. Suitable ranges of the values of x, y, and k are,

0.2,and3 5 k 4,

respectively. More preferably, the ranges of x and y are,

0 2 x 0.05 and 0.01 y g 0.2.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic model of the B-W type crystal structure.

FIG. 2 is a graph showing the relation between the concentration of a third element M and the critical temperature of the materials as cast of the Nb Ga, ,,M,, series.

FIG. 3 is a graph showing the relation between the concentration of Al and the critical temperature of the material as cast and as aged" of the Nb Ga, Al,, series.

FIG. 4 is a graph showing the relation between the concentration of M (M Mn, Sb or Y) and critical temperature of the material as aged of the Nb Ga, ,,M,, series.

FIG. 5 is a graph showing the relation between the concentration of Al the the critical temperature of the material cast and as aged" of the FIG. 6 is a graph showing the relation between the critical temperature and the aging temperature of Nb Ga Al FIG. 7 is a graph showing the H-Jc curve of several materials of this invention and that of previously known Nb Ga.

FIG. 8 is a graph showing the relation between the critical current density and the aging temperature of 3 o.s0 o.2o-

DESCRIPTION OF THE PREFERRED EMBODIMENTS A model (unit cell) of the B-W type crystal structure of the intermetallic compound represented by a chemical formula Nb Ga is shown in FIG. 1. It is characteristic of this crystal structure that it is constructed of a complex lattices structure comprising a body centered cubic crystal lattice consisting of Ga atoms and a chain like lattice consisting of Nb atoms in which each pair of Nb atoms are present in each of the faces of the body centered cubic lattice, three pairs of Nb atoms in three orthogonal faces of the body centered cubic lattice being orthogonal to each other.

In this superconducting material having the B-W type crystal structure, it is generally known that the critical temperature increases when the composition of the material approaches the stoichiometric composition and that the chains of Nb atoms form an extremely narrow d-band structure at the Fermi surface in the B-W type crystal structure.

Since in the d-band structure as aforesaid the density state of d-electron at the Fermi surface is increased, the critical temperature is thereby increased.

The present invention is based on the discovery by the inventors that when a part of either the Nb atoms or Ga atoms of the superconducting material Nb Ga having a B-W type crystal structure are substituted by the atoms of a transition metal having an atomic radius different from those of the atoms of the superconducting material so that the ratio between the average radius of the atoms in the Nb-sites and the average radius of the atoms in the Ga-sites approaches l, the B-W type crystal approaches the closest packed structure thereby facilitating the formation of the stable B-phase and providing a superconducting material having a high critical temperature.

More particularly, the present invention is based on the discovery by the present inventors that when the Ga atoms of the Nb Ga are partially substituted by the atoms of an aforementioned third element selected from the group consisting of Mn, Al, Sb and Y, a material having a higher critical temperature than the base compound Nb Ga can be provided.

Furthermore, the present invention is based on the discovery by the present inventors that when a suitable amount of Nb of the above-mentioned superconducting material is substituted by Ta in accordance with the present invention, the B-W type crystal structure approaches the closest packed structure and the material shows a more stable B-phase.

As raw materials, respective amounts of elements Nb, Ga, and a third element M (M Al, Sb, Mn, Y, I-If, B, Be, Si and Zr) each having a purity of 99 percent up were prepared, and weighed quantities of them for various desired values of x, y, and k of the chemical formula Nb Ga, ,,M,, were melted in an argon atmosphere in a plasma arc furnace, the melts were mixed and inverted several times to mix them uniformly, and the melts were solidified into pellets. The materials thus prepared were again melted using a levitation melting furnace, and then cast into a water-cooled copper mold to form a rod shape ingot (hereinafter referred to as an as cast specimen) of about 3 mm in diameter and about 30 mm long. Casting is conducted in an argon atmosphere.

The as cast specimen was muffled in an Nb foil and placed in a quartz tube, sealed in a high vacuum and then aged at a temperature of 600 to 1,200C for 24 to 360 hours. Generally, the high vacuum is about mmHg and preferably about 10 mmI-Ig or higher vacua.

The values of x, y, and k of the resulting intermetallic compounds were quantitatively determined by a chemical analysis, and the crystal structures and lattice constants thereof were examined by an X-ray powder diffraction method.

According to the result of the X-ray analysis, some of the as cast specimens showed X-ray diffraction patterns of the fiW type crystal structure, which patterns were observed to be broad. As the heat treatment proceeded, the patterns became sharp. Particularly effective heat treatment is done in a temperature range of from about 600 to about l,l00C for a period of time equal to at least about 24 hours and preferably more than 100 hours and in some cases longer, e.g. 360 hours.

From the result of the above-mentioned investigations, it has been confirmed that Nb Ga, ,,M,,, in which the values of y and k are selected within ranges of 0 y 0.2, and 3 k 4, respectively, results in a favorable fi-W type crystal structure by virtue of the above-mentioned heat treatment.

According to the present invention, more particularly, when the value ofy is within a range 0.0] y

0.2 and when the value of k is about 3, that is, the chemical composition comprises about 75 atomic percent of Nb, about 24.75 atomic percent of Ga, and about 0.25-5 atomic percent of the third element M, (M Mn, Sb, Al or Y), the materials have relatively high critical temperatures as compared with the previously known compound Nb Ga.

The superconductive materials according to the present invention provided in such a manner are relatively lower in the hardness than that of the prior art Nb AI base B-W type ternary intermetallic compounds, i.e. about 820 Kp/mm in Knoop hardness, and hence cracks hardly occur in the materials as cast. Thus, the materials according to the present invention are mechanically extremely superior to the prior art materials.

The critical temperature was measured by a conventional four-probe resistivity technique when a current density of l A/cm passed through a specimen of 30 mm long. The critical temperature was determined to be a temperature at which the resistivity of the specimen became one-half the difference between resistivities of the superconducting and normal states during the transition. The thermometer used in that measurement of the critical temperature is a germanium thermometer by Honeywell Co. in the US. which is calibrated at three temperatures of liquid helium, liquid hydrogen, and liquid nitrogen under an atmosphere.

FIG. 2 shows variations in the critical temperatures of Nb Ga, ,,M,, (M Al, Sb, Mn, Y, B, Be, Hf, Si and Zr) series ternary intermetallic compounds as cast. For M B, Be, Hf, Si, or Zr, the critical temperature rapidly lowers from the critical temperature of Nb Ga (about 15K) with the atomic concentration y within a range of0 to 0.2. On the contrary, for M A], Sb, Mn, or Y, the critical temperature thereof is higher than that of Nb Ga at the values of the atomic concentration y within a range of 0 to 0.3. A peak of the critical temperature exists within the range of y between 0.1 and 0.2.

FIG. 3 shows the relations between the critical temperatures of Nb Ga ,,Al,, series intermetallic compounds as cast and as aged (aging: 700C X 300 hours), where k is fixed at about 3.8, and the atomic concentration y of Al. FIG. 3 also shows the relation for the case where k is about 2.8. In the case ofk 3.8, two peaks of the critical temperature which are higher than those of the mother alloys Nb Ga (15K) and Nb Al (l8.lK) are present. The critical temperature of the ternary alloy is higher in the composition region of the atomic concentration y of Al of about 0.05 and higher than that of Nb Al (y 1.0). The peak value of the critical temperature nearer to the composition Nb Ga at y being about 0.15 is 20.6I(, and that nearer to the composition Nb Al at y z 0.9 is 19.lI(. In the case ofk z 2.8, the critical temperature monotonically increases with the atomic concentration in the region on the side of Nb Ga, and no peak of the critical temperature is present.

Thus, it has been found that for Nb Ga, ,,Al,, series ternary alloy with k 3 having the B-W type crystal structure a first peak of the critical temperature lies on the Nb Ga side, and a second peak lies on the Nb Al side. The compounds with k 3, having the B-W type crystal structure, has, on the Nb Ga side, no peak of the critical temperature and the critical temperature monotonically lowers with the increase in the concentration of gallium atoms.

Consequently, by replacing a part of the atoms in the Ga sites of Nb Ga (3 k 4) by Al atoms, superconducting materials of a stable BW type crystal structure can be provided which have the critical temperatures higher than those of the binary compounds Nb Al and Nb Ga, and have superconductive characteristics equivalent to and lower brittleness than the ternary compound Nb Al Ga having the known highest critical temperature. The region of composition at which such intermetallic compounds having the B-W type crystal structure and high critical temperatures are provided is 75 to 80 atomic percent for Nb, 1 to 24 atomic percent for Ga, and l to 24 atomic percent for Al.

FIG. 4 shows the relationship between the concentration of third element M of Nb Ga ,,M,, (M A1, Mn, Sb or Y) and the critical temperatures of as cast and as aged specimens provided under the same condition noted above. As seen from FIG. 4, the relation between the critical temperature and composition of these specimens is similar to that of the abovementioned Nb Ga, Al,,. The peak values of the critical temperature of the present as aged examples were I9.9K (M= Aluminum; y 0.15), 18.5K (M= Antimony; y 0.12), l8.lK (M Yttrium; y 0.07) and l7.8K (M Manganese; y 0.10).

Generally, in the present invention, the amount of Ta substituted for Nb is not more than about 5 atomic percent.

FIG. 5 shows the relation between the critical temperature and the atomic concentration y of A1 of the superconducting materials according to the present invention as cast and aged produced by substituting Ta atoms for a part of Nb atoms of Nb Ga, ,Al,,. The conditions for manufacturing these materials are similar to those for the above-described embodiments. The aging process is 700C X 30 hours. From these results it can be seen that by replacing a part of the atoms in the Nbsites with Ta atoms a material as aged having a relatively high critical temperature and a more stable B-W type crystal structure can be produced with a relatively short time aging.

FIG. 6 is an example of the relation between the critical temperature and the aging temperature of the superconducting material according to the present invention as aged. Though this example is for Nb Ga Al other superconducting materials according to the present invention have also similar relations to that of FIG. 6. According to the curve of FIG. 6, the temperature suitable for aging the superconducting materials according to the present invention is at a range of from 600 to 1,000C, in particular 700 to I,000C, and more preferably 750 to 950C. As to the time duration for aging, even 1 hour or so already has a marked ef fect, and the longer the time duration is, the higher the critical temperature of the material as aged is. Though there is thus no upper bound of the time duration of aging, at a range of from about 300 up to 500 hours the rise of the critical temperature is substantially saturated. For this reason, from the standpoint of economy of time the aging time is preferably less than 500 hours, in particular between 100 and 300 hours.

The superconducting materials according to the present invention is excellent not only in having high critical temperatures, but also in having high critical current characteristics.

FIG. 7 shows a comparison between the characteristies as regards the critical current densities (.Ic) of the superconducting materials according to the present in- FIG. 8 shows the exemplary relation between the critical current density and the aging temperature of the superconducting material according to the present invention as aged in which Nb3Ga A1 is taken as an example. The critical current density was measured at 4.2I(, and the intensity of the magnetic field orthogonal to the current was kOe. The aging time was 1 hour. It is evident from FIG. 8 that the superconducting material according to the present invention has a critical current density higher than 10 A/cm at temperatures of from 500 to l,l00C and under a magnetic field of 80 kOe, and in particular has a very high critical current density of 4 X 10 A/cm at an aging temperature of about 850C.

All of the above described superconducting material embodying the present invention were manufactured by melting in a plasma arc furnace. For putting these superconducting materials having excellent characteristics into practical use, it is desirable to form them into the shape of a composite tape by depositing a layer of the superconducting material on a predetermined substrate, for example, a niobium substrate by a vapor phase reaction method, a melt clad diffusion method or the like.

When a composite superconducting tape is to be produced by a vapor reaction method, halides of the component elements, for example, NbCl GaCl and AICI; vapors are passed over a tape of stainless steel or other refractory alloys heated to 800 to 1,200C to be subjected to hydrogen reduction. Then, by the chemical reaction represented by the chemical reaction formula,

a layer of a compound Nb Ga, ,,Al,, (3 k 4) is deposited on the tape to a thickness of several microns. Of course, in order to eliminate the instability of the critical current on the low magnetic field side, it may be well to cover the compound layer with an ordinary conducting metal having a high electric conductivity and a large thermal diffusion coefficient such as copper or silver by plating or cladding as is commonly known. It is also possible to grow an Nb Ga, ,,Al,, layer from the vapor phase by employing reaction vapors of iodides or bromides of Nb, Ga and Al.

When an immersion clad diffusion method is employed, a sufficiently clear Nb plate or an Nb-Ta alloy plate of an appropriate dimension is passed in a thermostat bath of molten Ga-Al alloy placed in a vacuum or in an inert atmosphere such as argon atmosphere to clad the plate or tape on its both surfaces with a predetermined Ga-AI alloy to a thickness of several microns, and then passed through a high temperature furnace maintained at a temperature of from 800 to 1,600C in a vacuum or in an inert atmosphere such as argon atmosphere to subject it to heat treatment, so that a layer of Nb Ga, ,,Al,, or (NbTa) Ga, ,,Al,, is formed on the tape by diffusion. It is also possible to form an intermediate phase having relatively high compompositions of Ga and Al by diffusion at low temperatures followed by the formation of Nb Ga, ,,Al,, or (NbTa) Ga, ,,Al,, by heat treatment at relatively low temperatures for diffusron.

As is evident from the above detailed description, the superconducting materials according to the present invention of a novel composition of Nb-Ga-M or Nb-Ta- Ga-M (M Al, Sb, Mn or Y) and of B-W type crystal structure have a critical temperature of 20.6K at a maximum and are relatively easy to manufacture. Consequently, these superconducting materials can be used with a liquid hydrogen coolant at an ordinary pressure or reduced pressures. Such a mitigation of cooling conditions greatly improve the economy of superconductive appliances.

We claim:

1. A superconducting material consisting essentially of an intermetallic compound having a /3 W type crystal structure, said compound being represented by the formula:

1-.z- .r)k lu u where M is an element selected from the group consisting of Mn, Sb, Y, and Al, and the values ofx and y are respectively, and the value of k is about 3.8.

2. A superconducting material consisting essentially of an intermetallic compound having a B-W type crys- 0.07 0.2, and

4. A superconducting material according to claim 1, wherein the value ofk is 3.8, and wherein the superconducting material has a critical temperature of about 20.6K.

5. A superconducting material according to claim 4, wherein the value of x is 0 and the value of y is about 0.15.

6. A superconducting material according to claim 1, wherein said intermetallic compound is Nb Ga, Al where 0 y 0.2.

7. A superconducting material according to claim 1, wherein said intermetallic compound is Nb Ga, ,Sb,, where O y 0.2.

8. A superconducting material according to claim 1, wherein said intermetallic compound is Nb Ga, Mn,, where 0 y 0.2.

9. A superconducting material according to claim 1, wherein said intermetallic compound is Nb Ga, ,,Y,, where O y 0.2.

10. A superconducting material according to claim 6, wherein y has a value of about 0.15 and the superconducting material has a critical temperature of about 20.6K.

1 l. A superconducting material consisting essentially of an intermetallic compound having a B-W type crystal structure, said compound being represented by the formula:

1 z)k -lu u where M is an element selected from the group consisting of Mn, Sb, and Y, and the values of x, y and k are, respectively, 

2. A superconducting material consisting essentially of an intermetallic compound having a Beta -W type crystal structure, said compound being represented by the formula: (Nb1 xTax)kGa1 yMy where M is an element selected from the group consisting of Mn, Sb, Y and Al, and the values of x, y, and k are 0 < or = x < or = 0.07 0 < y < or = 0.2, and about 3.8 < or = k < or = 4.0, respectively.
 3. A superconducting material according to claim 2, wherein the values of x, y, and k are 0 < or = x < or = 0.05, 0.025 < or = y < or = 0.15, and k 3.8, respectively.
 4. A superconducting material according to claim 1, wherein the value of k is 3.8, and wherein the superconducting material has a critical temperature of about 20.6*K.
 5. A superconducting material according to claim 4, wherein the value of x is 0 and the value of y is about 0.15.
 6. A superconducting material according to claim 1, wherein said intermetallic compound is Nb3.8Ga1 yAly, where 0 <y < or = 0.2.
 7. A superconducting material according to claim 1, wherein said intermetallic compound is Nb3.8Ga1 ySby where 0<y < or = 0.2.
 8. A superconducting material according to claim 1, wherein said intermetallic compound is Nb3.8Ga1 yMny where 0<y < or = 0.2.
 9. A superconducting material according to claim 1, wherein said intermetallic compound is Nb3.8Ga1 yYy where 0<y < or = 0.2.
 10. A superconducting material according to claim 6, wherein y has a value of about 0.15 and the superconducting material has a critical temperature of about 20.6*K.
 11. A superconducting material consisting essentially of an intermetallic compound having a Beta -W type crystal structure, said compound being represented by the formula: (Nb1 xTax)kGa1 yMy where M is an element selected from the group consisting of Mn, Sb, and Y, and the values of x, y and k are, respectively, 0 < or = x < or = 0.07 0 < or = y < or = 0.2 3 < or = k < or =
 4. 